Automatic transmission

ABSTRACT

An automatic transmission is provided, which includes a friction engaging element. The friction engaging element includes a plurality of friction plates disposed inside a transmission case, a piston configured to cause the plurality of friction plates to be engaged with each other, and a spring configured to bias the piston. The spring includes a first spring and a second spring with a longer free length than the first spring, and the first spring and the second spring are aligned in circumferential directions of the transmission case.

TECHNICAL FIELD

The present disclosure relates to an automatic transmission mounted on avehicle, and belongs to a technical field of the automatic transmissionfor vehicles.

BACKGROUND OF THE DISCLOSURE

An automatic transmission mounted on a vehicle is provided with aplurality of planetary gear sets (planetary gear mechanisms), and aplurality of friction engaging elements, such as a clutch and a brake.The automatic transmission is constructed so that a plurality offriction engaging elements are selectively engaged with a hydrauliccontrol to switch between power transmission paths via respectiveplanetary gear sets, thereby achieving a plurality of forward gearratios and normally one reverse gear ratio.

In recent years, a torque converter tends to be eliminated from theautomatic transmission because the transmission is demanded, forexample, to have more selectable gear ratios, and to reduce the weight.Without the torque converter, the vehicle can smoothly start travelingwithout an engine stall by carrying out a slip control of at least onefriction engaging element which is engaged at a first gear.

If carrying out the slip control of the friction engaging element whichis engaged at the first gear when the vehicle starts traveling, sincethe brake of which a hydraulic chamber does not rotate is better incontrollability when engaged, than the clutch of which a hydraulicchamber rotates, the slip control of the brake may be carried out.

Among the brakes constructed in this way, it is known that a pistonwhich causes a friction plate to be engaged is biased and moved in theengaging direction by a spring in order to improve the response when thevehicle starts traveling.

For example, JP2017-150533A discloses a brake in which a piston whichcauses a plurality of friction plates to be engaged is biased by a firstspring and a second spring in the engaging direction from the releasedposition to a first position a given distance from the releasedposition, is biased only by the second spring from the first position toa second position where the plurality of friction plates become in azero clearance state, and is biased by the hydraulic pressure forengagement from the second position to an engaged position to cause theplurality of friction plates to be engaged.

FIG. 17 illustrates a cross-sectional view of such a brake 200 in theautomatic transmission. The brake 200 includes a plurality of frictionplates 203 disposed between an inner stationary member 201 coupled to atransmission case and an outer rotary member 202 coupled to a givenrotary member, and a piston 206 fitted into a cylinder 205 which isformed by an outer cylindrical part 204 a, a flange part 204 b, and aninner cylindrical part 204 c of a housing 204 which are parts of thetransmission case.

The brake 200 also includes a hydraulic chamber 207 for engagement towhich hydraulic fluid for engagement which biases the piston 206 in theengaging direction is supplied, and a hydraulic chamber 208 for releasewhich is disposed at the opposite side of the hydraulic chamber 207 withthe piston 206 therebetween, and to which hydraulic fluid for releasewhich biases the piston 206 in the releasing direction is supplied.

In the hydraulic chamber 207 for engagement, a first spring 209 and asecond spring 210 which bias the piston 206 in the engaging directionare disposed. The second spring 210 is disposed inside a groove portion204 d formed in the outer cylindrical part 204 a of the housing 204, andthe first spring 209 is disposed radially inward of the second spring210.

When engaging the brake 200, if the hydraulic pressure for release isreleased from a state in which the hydraulic pressure for engagement isreleased from the hydraulic chamber 207 for engagement, and hydraulicpressure for release is supplied to the hydraulic chamber 208 forrelease to move the piston 206 to the released position where the firstspring 209 and the second spring 210 are compressed, the piston 206 isbiased by the first spring 209 and the second spring 210 to be moved tothe first position where is the given distance away from the releasedposition in the engaging direction.

When the piston 206 reaches the first position, the piston 206 is thenbiased only by the first spring 209 to be moved from the first positionto the second position where the plurality of friction plates 203 becomein the zero clearance state. After the piston 206 reaches the secondposition, when the hydraulic pressure for engagement is supplied, thepiston 206 is then biased by the hydraulic pressure for engagement to bemoved to the engaged position where the plurality of friction plates 203are engaged.

On the other hand, when releasing the brake 200, if the hydraulicpressure for engagement is released and the hydraulic pressure forrelease is supplied, from a state in which the hydraulic pressure forrelease is released from the hydraulic chamber 208 for release, and thehydraulic pressure for engagement is supplied to the hydraulic chamber207 for engagement to move the piston 206 to the engaged position, thepiston 206 is biased in the releasing direction and the piston 206 ismoved to the released position where the first spring 209 and the secondspring 210 are compressed.

In the brake 200, since the biasing force of the second spring 210 isset larger than the biasing force of the first spring 209, the piston206 can be moved with sufficient response by the first spring 209 andthe second spring 210 from the released position to the first position,and can then be moved with sufficient accuracy by the first spring 209from the first position to the second position.

However, the automatic transmission provided with the brake disclosed inJP2017-150533A has a structure with a large radial dimension in whichthe first spring which biases the plurality of friction plates in theengaging direction, and the second spring of which the biasing force isset larger than that of the first spring are disposed serially in theradial directions.

Therefore, in such an automatic transmission, shortening the radialdimension to downsize the automatic transmission is desired, when thetransmission is structured to have as the biasing member the firstbiasing member which biases the piston in the engaging direction, andthe second spring of which the biasing force is set larger than that ofthe first spring.

SUMMARY OF THE DISCLOSURE

Therefore, one purpose of the present disclosure is to provide anautomatic transmission provided with a friction engaging element havinga biasing member (spring) which biases a piston, which can be radiallydownsized when the biasing member includes a first biasing member (firstspring) and a second biasing member (second spring).

According to one aspect of the present disclosure, an automatictransmission is provided which includes a friction engaging element. Thefriction engaging element includes a plurality of friction platesdisposed inside a transmission case, a piston configured to cause theplurality of friction plates to be engaged with each other, and a springconfigured to bias the piston, the spring including a first spring and asecond spring with a longer free length than the first spring, and thefirst spring and the second spring being aligned in circumferentialdirections of the transmission case.

According to this configuration, compared to a case where first springand the second spring are not aligned in the circumferential directions,the automatic transmission as described above can be shortened in radialdimensions and thereby it can be radially downsized.

Since the spring may include the first and second springs, the pistoncan be moved with sufficient response from a released position to animmediately-before-contact position by the first spring, in addition tothe second spring, and the piston can be moved with sufficient accuracyfrom the immediately-before-contact position to a zero clearanceposition by the second spring, and thereby the piston can be moved withsufficient response and with sufficient accuracy to the zero clearanceposition. In particular, the first spring may be configured to exert alarger biasing force than the second spring, and thereby the piston canbe moved with sufficient response and with sufficient accuracy to thezero clearance position.

Since the piston can be moved with sufficient accuracy to the zeroclearance position, a shock caused by the piston being located on theengagement side of the zero clearance position to press the frictionplates, and a slowing of the response caused by the piston being locatedon the releasing side of the zero clearance position, can be prevented.

The automatic transmission may further include a hydraulic chamber towhich hydraulic fluid is supplied, the hydraulic fluid biasing thepiston. The hydraulic chamber may include a hydraulic chamber forengagement to which hydraulic fluid to bias the piston in an engagingdirection is supplied to cause the plurality of friction plates to beengaged with each other, and a hydraulic chamber for release disposed atan opposite side of the piston from the hydraulic chamber forengagement. The hydraulic chamber for release may be supplied withhydraulic fluid to bias the piston in a releasing direction oppositefrom the engaging direction. The hydraulic chamber for release may beformed to have a smaller outer diameter than the hydraulic chamber forengagement. A biasing force receiving member configured to receive thebiasing force of the spring may be coupled to the piston and disposed atan outer circumferential side of the hydraulic chamber for release.

According to this configuration, the biasing force receiving member canbe coupled to the piston at the outer circumferential side of thehydraulic chamber for release of which the outer diameter is formedsmaller than the hydraulic chamber for engagement, thereby effectivelyutilizing the space at the outer circumferential side of the hydraulicchamber for release.

Further, the hydraulic chamber may be disposed so as to overlap witheach of the first spring and the second spring in a radially internaland external relationship.

According to this configuration, compared to a case where the hydraulicchamber is not overlapped with the first and second springs in aradially internal and external relationship, the automatic transmissioncan be shortened in axial dimensions and thereby it can be axiallydownsized.

Further, the automatic transmission may further include a first retainerplate holding a first end part of the first spring and a first end partof the second spring, and a second retainer plate, disposed so as to beaxially separated from the first retainer plate, holding a second endpart of the first spring, and having an insertion hole into which thesecond spring is inserted so that a second end part of the second springis projectable in a direction opposite from the first retainer plate.

According to this configuration, since the piston is biased by thesecond spring projected on the second retainer plate to the directionopposite from the first retainer plate from theimmediately-before-contact position to the zero clearance position, thepiston can be moved with a comparatively simple structure from thereleased position to the immediately-before-contact position by thefirst and second springs, and the piston can be moved from theimmediately-before-contact position to the zero clearance position bythe second spring.

According to another aspect of the present disclosure, an automatictransmission is provided, which includes a friction engaging element.The friction engaging element includes a plurality of friction platesdisposed inside a transmission case, a piston configured to cause theplurality of friction plates to be engaged with each other, and a springconfigured to bias the piston in an engaging direction to cause theplurality of friction plates to be engaged with each other, the springincluding a first spring and a second spring with a longer free lengththan the first spring, and the first spring and the second spring beingaligned in circumferential directions of the transmission case.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically illustrating an automatic transmissionaccording to one embodiment of the present disclosure.

FIG. 2 is an engagement table of friction engaging elements of theautomatic transmission.

FIG. 3 is a cross-sectional view illustrating a brake of the automatictransmission, and its periphery.

FIG. 4 is another cross-sectional view illustrating the brake and itsperiphery.

FIG. 5 is another cross-sectional view illustrating the brake and itsperiphery.

FIG. 6 is another cross-sectional view illustrating the brake and itsperiphery.

FIG. 7 is another cross-sectional view illustrating the brake and itsperiphery.

FIG. 8 is a perspective view illustrating an assembled state of a hubmember, a biasing unit, an oil channel forming member, and a piston ofthe brake.

FIG. 9 is a perspective view illustrating an assembled state of the hubmember, the biasing unit, and the oil channel forming member of thebrake.

FIG. 10 is a perspective view illustrating an assembled state of the hubmember and the biasing unit of the brake.

FIG. 11 is a perspective view illustrating the biasing unit.

FIG. 12 is a cross-sectional view illustrating the biasing unit takenalong a line Y12-Y12 in FIG. 11.

FIG. 13 is a cross-sectional view illustrating the brake in a releasedstate.

FIG. 14 is a cross-sectional view illustrating the brake in animmediately-before-contact state.

FIG. 15 is a cross-sectional view illustrating the brake in a zeroclearance state.

FIG. 16 is a cross-sectional view illustrating the brake in an engagedstate.

FIG. 17 is a cross-sectional view illustrating a brake of a conventionalautomatic transmission.

DETAILED DESCRIPTION OF THE DISCLOSURE

Hereinafter, one embodiment of the present disclosure is described withreference to the accompanying drawings.

FIG. 1 is a view schematically illustrating an automatic transmissionaccording to one embodiment of the present disclosure. An automatictransmission 10 has, in a transmission case 11, an input shaft 12coupled to a driving source and disposed at the driving source side(left side in this figure), and an output shaft 13 disposed at theopposite side of the driving source side (anti-driving-source side, orright side in this figure). The automatic transmission 10 is of alongitudinal type for a front-engine rear-drive (FR) vehicle in whichthe input shaft 12 and the output shaft 13 are disposed coaxially.

On the axial center of the input shaft 12 and the output shaft 13,first, second, third, and fourth planetary gear sets (hereinafter,simply referred to as “the first, second, third, and fourth gear sets”)PG1, PG2, PG3, and PG4 are disposed in this order from the drivingsource side.

In the transmission case 11, a first clutch CL1 is disposed at thedriving source side of the first gear set PG1, a second clutch CL2 isdisposed at the driving source side of the first clutch CL1, and a thirdclutch CL3 is disposed at the driving source side of the second clutchCL2. Moreover, a first brake BR1 is disposed at the driving source sideof the third clutch CL3, and a second brake BR2 is disposed at thedriving source side of the third gear set PG3 and at theanti-driving-source side of the second gear set PG2.

Each of the first, second, third, and fourth gear sets PG1, PG2, PG3,and PG4 is of a single pinion type in which a pinion supported by acarrier directly meshes with sun gears and a ring gear. The first,second, third, and fourth gear sets PG1, PG2, PG3, and PG4 have sungears S1, S2, S3, and S4, ring gears R1, R2, R3, and R4, and carriersC1, C2, C3, and C4, as rotary members, respectively.

The first gear set PG1 is of a double sun gear type in which the sungear S1 is divided into two in the axial directions. The sun gear S1 hasa first sun gear S1 a disposed at the driving source side, and a secondsun gear S1 b disposed on the anti-driving-source side. The first andthe second sun gears S1 a and S1 b have the same number of teeth, andmesh with the same pinion supported by the carrier C1. Thus, the firstand the second sun gears S1 a and S1 b always rotate together.

In this automatic transmission 10, the sun gear S1 of the first gear setPG1 (specifically, the second sun gear S1 b) is always coupled to thesun gear S4 of the fourth gear set PG4, the ring gear R1 of the firstgear set PG1 is always coupled to the sun gear S2 of the second gear setPG2, the carrier C2 of the second gear set PG2 is always coupled to thecarrier C4 of the fourth gear set PG4, and the carrier C3 of the thirdgear set PG3 is always coupled to the ring gear R4 of the fourth gearset PG4.

The input shaft 12 is always coupled to the carrier C1 of the first gearset PG1 via the first sun gear S1 a and the second sun gear S1 b, andthe output shaft 13 is always coupled to the carrier C4 of the fourthgear set PG4.

The first clutch CL1 is disposed between the input shaft 12 and thecarrier Cl of the first gear set PG1, and the sun gear S3 of the thirdgear set PG3, and connects and disconnects these gear sets. The secondclutch CL2 is disposed between the ring gear R1 of the first gear setPG1 and the sun gear S2 of the second gear set PG2, and the sun gear S3of the third gear set PG3, and connects and disconnects these gear sets.The third clutch CL3 is disposed between the ring gear R2 of the secondgear set PG2 and the sun gear S3 of the third gear set PG3, and connectsand disconnects these gear sets.

The first brake BR1 is disposed between the transmission case 11 and thesun gear S1 of the first gear set PG1 (specifically, the first sun gearS1 a), and connects and disconnects these gears. The second brake BR2 isdisposed between the transmission case 11 and the ring gear R3 of thethird gear set PG3, and connects and disconnects these gears.

With the above structure, the automatic transmission 10 selectivelycombines the engaged states of the first clutch CL1, the second clutchCL2, the third clutch CL3, the first brake BR1, and the second brake BR2to form first to eight gears for a D-range (forward driving range) and areverse gear for an R-range, as illustrated in FIG. 2.

In this automatic transmission 10, a slip control is carried out for thesecond brake BR2 which engages at the first gear when the vehicle startstraveling, and thereby the second brake BR2 corresponds to a frictionengaging element of the automatic transmission according to the presentdisclosure. Below, this brake BR2 is described.

FIG. 3 is a cross-sectional view of the brake of the automatictransmission and its periphery, FIG. 4 is another cross-sectional viewof the brake and its periphery, and FIGS. 5-7 are other cross-sectionalviews of the brake and its periphery. FIGS. 3-7 illustrate crosssections of the brake and its periphery, taken along lines Y3-Y3, Y4-Y4,Y5-Y5, Y6-Y6, and Y7-Y7 in FIG. 9, respectively, as will be described indetail.

As illustrated in FIGS. 3-7, the brake BR2 is accommodated in thetransmission case 11 formed in a substantially cylindrical shape. Thebrake BR2 is disposed at an outer circumferential surface of a powertransmission member 14 which is coupled to the sun gear S3 of the thirdgear set PG3 so as to be integrated with one of a pair of inner andouter rotary members of the first, second, and third clutches CL1, CL2,and CL3.

The power transmission member 14 is disposed at an outer circumferentialsurface of a power transmission member 15 which couples the carrier C2of the second gear set PG2 to the carrier C4 of the fourth gear set PG4,and the power transmission member 15 is disposed at an outercircumferential surface of a power transmission member 16 which couplesthe sun gear Si of the first gear set PG1 (specifically, the second sungear S1 b) to the sun gear S4 of the fourth gear set PG4.

The brake BR2 includes a hub member 20 coupled to the transmission case11, a drum member 30 which is disposed on the anti-driving-source sideof the hub member 20 and is coupled to the ring gear R3 of the thirdgear set PG3, a plurality of friction plates 40 placed in line in theaxial directions between the hub member 20 and the drum member 30, and apiston 50 which is disposed on the anti-driving-source side of theplurality of friction plates 40 and causes the plurality of frictionplates 40 to be engaged.

The brake BR2 has a hydraulic chamber 60 to which hydraulic fluid whichbiases the piston 50 is supplied, and the hydraulic chamber 60 includesa hydraulic chamber 61 for engagement to which hydraulic fluid forengagement which biases the piston 50 in the engaging direction issupplied, and a hydraulic chamber 62 for release which is disposed atthe opposite side of the piston 50 from the hydraulic chamber 61 forengagement, and to which hydraulic fluid for release which biases thepiston 50 in the releasing direction is supplied.

As illustrated in FIG. 5, the brake BR2 also has an oil channel formingmember 70 which forms a supply oil channel for engagement which supplieshydraulic fluid to the hydraulic chamber 61 for engagement, and the oilchannel forming member 70 is disposed on the anti-driving-source side ofthe piston 50 and is coupled to the anti-driving-source side of the hubmember 20.

As illustrated in FIG. 7, the brake BR2 also has a biasing unit 80provided with a biasing member 81 which biases the piston 50, and thebiasing unit 80 includes, as the biasing member 81, first springs 82 andsecond springs 83 which bias the piston 50 in the engaging direction, asfirst biasing members and second biasing members. That is, the biasingmember 81 may be referred to as a spring.

FIG. 8 is a perspective view illustrating an assembled state of the hubmember, the biasing unit, the oil channel forming member, and the pistonof the brake, and FIG. 9 is a perspective view illustrating an assembledstate of the hub member, the biasing unit, and the oil channel formingmember of the brake, FIG. 10 is a perspective view illustrating anassembled state of the hub member and the biasing unit of the brake,FIG. 11 is a perspective view illustrating the biasing unit, and FIG. 12is a cross-sectional view illustrating the biasing unit taken along aline Y12-Y12 in FIG. 11.

As illustrated in FIGS. 3-10, the hub member 20 includes a vertical wallpart 21 which extends in a direction perpendicular to the axialdirections of the transmission case 11 and having a substantiallyannular shape, as well as a first cylinder part 22, a second cylinderpart 23, and a third cylinder part 24 which extend to theanti-driving-source side from the vertical wall part 21 and each havinga substantially cylindrical shape.

The first cylinder part 22 extends axially from the radially center sideof the vertical wall part 21, the second cylinder part 23 extendsaxially from the vertical wall part 21, at a location radially inward ofthe first cylinder part 22, and the third cylinder part 24 extendsaxially from the vertical wall part 21, at a location radially inward ofthe second cylinder part 23. The first cylinder part 22 is longer in theaxial directions than the second cylinder part 23, and is shorter in theaxial directions than the third cylinder part 24.

The hub member 20 is fitted into a groove portion 11 a for the hubmember which is formed in an inner circumference surface of thetransmission case 11 to have a substantially cylindrical shape accordingto the shape of the vertical wall part 21, and is prevented by a snapring 17 from coming out to the driving source side. The hub member 20 iscoupled to the transmission case 11 by fixing the vertical wall part 21to the transmission case 11 by using a rotation stop pin (notillustrated).

A spline 22 a is formed in an outer circumferential surface of the firstcylinder part 22 of the hub member 20 so that the first cylinder part 22constitutes an inner stationary member coupled to the transmission case11. Stationary-side friction plates 41 which constitute the frictionplates 40 spline-engage with the spline 22 a. The second cylinder part23 and the third cylinder part 24 constitute a cylinder 62 a of thehydraulic chamber 62 for release together with the vertical wall part21.

The drum member 30 includes a cylinder part 31 which is disposed at theouter circumferential side of the first cylinder part 22 of the hubmember 20 so as to oppose the first cylinder part 22 and extends axiallyto have a substantially cylindrical shape, and a vertical wall part 32which extends in a direction perpendicular to the axial directions ofthe transmission case 11 from the anti-driving-source side of thecylinder part 31 radially inwardly to have a substantially disk shape.

The vertical wall part 32 of the drum member 30 is coupled to the ringgear R3. A spline 31 a is formed in an inner circumference surface ofthe cylinder part 31 of the drum member 30 so that the cylinder part 31constitutes the outer rotary member coupled to the ring gear R3 as therotary member. Rotary-side friction plates 42 which constitute thefriction plates 40 spline-engage with the spline 31 a. Thestationary-side friction plates 41 and the rotary-side friction plates42 are disposed alternately in the axial directions.

The piston 50 is disposed between the hub member 20 and the drum member30, in more detail, between the first cylinder part 22 of the hub member20 and the cylinder part 31 of the drum member 30, and is slidablyfitted onto the outer circumferential side of the third cylinder part 24of the hub member 20. The piston 50 is prevented by a snap ring 18 fromcoming out to the anti-driving-source side.

The piston 50 includes a pressing part 51 which is formed annularly andis provided at the outer circumferential side of the piston 50 to pressthe friction plates 40, a hydraulic chamber forming part 52 forengagement which is provided at the inner circumferential side and formsthe hydraulic chamber 61 for engagement, and a coupling part 53 whichcouples the pressing part 51 to the hydraulic chamber forming part 52for engagement, and extends radially.

The pressing part 51 is disposed on the anti-driving-source side of thefriction plates 40, the hydraulic chamber forming part 52 for engagementis disposed radially inward of the first cylinder part 22 of the hubmember 20, and the coupling part 53 is provided so as to be coupled tothe hydraulic chamber forming part 52 for engagement through theanti-driving-source side of the first cylinder part 22 of the hub member20 from the pressing part 51. The pressing part 51 and the hydraulicchamber forming part 52 for engagement are provided so as to project tothe driving source side from the coupling part 53.

As illustrated in FIGS. 5 and 6, the oil channel forming member 70 isdisposed on the anti-driving-source side of the piston 50. The oilchannel forming member 70 is fitted onto the outer circumferential sideof the third cylinder part 24 of the hub member 20, and is coupled tothe anti-driving-source side of the first cylinder part 22 of the hubmember 20.

The oil channel forming member 70 includes bonding parts 71 which areprovided at the outer circumferential side, and are coupled to theanti-driving-source side of the first cylinder part 22 of the hub member20, a hydraulic chamber forming part 72 for engagement which is providedat the inner circumferential side and is disposed on theanti-driving-source side of the piston 50 to form the hydraulic chamber61 for engagement, and coupling parts 73 which couple the bonding parts71 to the hydraulic chamber forming part 72 for engagement, and extendradially.

The hydraulic chamber forming part 72 for engagement is formed annularlyand has a given thickness, and as illustrated in FIG. 3, it is fittedinto a space between the third cylinder part 24 of the hub member 20 andthe outer circumferential side of the hydraulic chamber forming part 52for engagement of the piston 50. The hydraulic chamber 61 for engagementis comprised of the hydraulic chamber forming part 52 for engagement ofthe piston 50, the hydraulic chamber forming part 72 for engagement ofthe oil channel forming member 70, and the third cylinder part 24 of thehub member 20.

As illustrated in FIGS. 4 and 6, the bonding parts 71 are formed so asto have the thickness less than that of the hydraulic chamber formingpart 72 for engagement, and are provided so as to overlap in the axialdirections with the anti-driving-source side of the hydraulic chamberforming part 72 for engagement. As illustrated in FIGS. 8 and 9, eachbonding part 71 is formed in an arc shape. The oil channel formingmember 70 is provided with the plurality of bonding parts 71 (in thisembodiment, three bonding parts 71) in the circumferential directions,which separate substantially equally from each other in thecircumferential directions.

Each bonding part 71 is formed with a bolt insertion hole 71 a intowhich a fastening bolt 74 is inserted as a fastening member, and a boltaccommodation hole 71 b in which a head 74 a of the fastening bolt 74 isaccommodated. The oil channel forming member 70 is coupled to theanti-driving-source side of the first cylinder part 22 by threadedlyengaging through the bolt insertion holes 71 a the fastening bolts 74with threaded holes 22 b formed on the anti-driving-source side of thefirst cylinder part 22 of the hub member 20. As the fastening bolts 74,a sealed bolt in which an outer circumferential surface of a thread part74 b is covered with a sealant is used.

Each coupling part 73 of the oil channel forming member 70 hassubstantially the same thickness as the bonding part 71, and asillustrated in FIGS. 8 and 9, it extends radially inward from thecircumferentially center side of the bonding part 71 and is coupled tothe hydraulic chamber forming part 72 for engagement.

As illustrated in FIG. 9, one of the bonding parts 71 disposed at thelower side of the transmission case 11 is coupled to the first cylinderpart 22 by using the two fastening bolts 74 at both sides in thecircumferential directions, and each of other two bonding parts 71disposed at the upper side of the transmission case 11 is coupled to thefirst cylinder part 22 by using one fastening bolt 74 at the center inthe circumferential directions.

Cutoff parts 53 a are formed in the coupling part 53 of the piston 50 sothat the coupling part 53 is cutoff in substantially the same shape asthe bonding parts 71 and the coupling parts 73 of the oil channelforming member 70. The oil channel forming member 70 is disposed withina radial range of the piston 50, and is disposed at a position where thebonding parts 71 and the coupling parts 73 of the oil channel formingmember 70 overlap with the coupling part 53 of the piston 50 in theaxial directions by being fitted into the cutoff parts 53 a of thecoupling part 53 of the piston 50. Thus, the automatic transmission canbe radially downsized by disposing the oil channel forming member 70within the radial range of the piston 50.

As illustrated in FIG. 3, the hydraulic chamber forming part 52 forengagement of the piston 50 includes an outer cylinder part 52 a whichis fitted onto the outer circumferential side of the hydraulic chamberforming part 72 for engagement of the oil channel forming member 70 andextends axially, an oil-pressure receiving part 52 b for engagementwhich extends radially inward from the driving source side of the outercylinder part 52 a, and an inner cylinder part 52 c which extends to theanti-driving-source side from the radially inward of the oil-pressurereceiving part 52 b for engagement, is fitted into the third cylinderpart 24 of the hub member 20, and extends axially.

In this automatic transmission 10, the hydraulic chamber 60 is disposedradially inward of the first cylinder part 22 of the hub member 20, andthe hydraulic chamber 61 for engagement and the hydraulic chamber 62 forrelease are disposed radially inward of the first cylinder part 22.

As described above, the hydraulic chamber 61 for engagement is formed bythe hydraulic chamber forming part 52 for engagement of the piston 50,the hydraulic chamber forming part 72 for engagement of the oil channelforming member 70, and the third cylinder part 24 of the hub member 20.The inner cylinder part 52 c of the piston 50 is prevented by the snapring 18 from coming out to the anti-driving-source side.

As illustrated in FIG. 3, a radially inward part of the oil-pressurereceiving part 52 b for engagement of the piston 50 is bulged to thedriving source side into a substantially channel shape in the crosssection to form a bulged part 52 d. The bulged part 52 d is slidablyfitted into the cylinder 62 a of the hub member 20 through seal members63 and 64. The hydraulic chamber 62 for release is formed by the bulgedpart 52 d of the piston 50 and the cylinder 62 a of the hub member 20.

In this automatic transmission 10, the hydraulic chamber 62 for releasehas a smaller outer diameter than the hydraulic chamber 61 forengagement, and a biasing force receiving member 90 is disposed at theouter circumferential side of the hydraulic chamber 62 for release. Thebiasing force receiving member 90 is coupled to the piston 50 andreceives a biasing force caused by the biasing member 81 of the biasingunit 80. Thus, the space at the outer circumferential side of thehydraulic chamber 62 for release can be used effectively.

As illustrated in FIG. 7, the biasing force receiving member 90 isformed annularly, and includes a radially extended part 91 extendingradially between the first cylinder part 22 and the second cylinder part23 of the hub member 20, and an axially extended part 92 extendingaxially to the anti-driving-source side from the radially inward of theradially extended part 91.

The biasing force receiving member 90 is coupled to the piston 50 bycoupling the anti-driving-source side of the axially extended part 92 toa part of the oil-pressure receiving part 52 b for engagement of thepiston 50 radially outward of the bulged part 52 d. The biasing unit 80provided with the biasing member 81 is attached between the biasingforce receiving member 90 (specifically, the radially extended part 91)and the oil channel forming member 70.

As illustrated in FIGS. 11 and 12, the biasing unit 80 includes thefirst springs 82 and the second springs 83 which extend axially, a firstretainer plate 84 which holds one ends of the first springs 82 and thesecond springs 83 at the anti-driving-source side, and a second retainerplate 85 which is axially separated from the first retainer plate 84 andholds the other end of the first springs 82 at the driving source side.

The first retainer plate 84 is formed annularly, and is provided with afirst spring guide part 84 a and a second spring guide part 84 b whichproject cylindrically to the driving source side and to which the firstsprings 82 and the second springs 83 are attached. The first springs 82and the second springs 83 are disposed at different circumferentialpositions but radially overlap with each other. Thus, the automatictransmission can be shortened in the radial dimensions. In thisautomatic transmission 10, the first spring 82 is disposed at both sidesof each of the six second springs 83 in the circumferential directions.

The second retainer plate 85 is formed substantially symmetry in theaxial directions to the first retainer plate 84. The second retainerplate 85 includes a first spring guide part 85 a which projectscylindrically to the anti-driving-source side and to which the firstsprings 82 are attached. The second retainer plate 85 is formed withinsertion holes 85 b into which the second springs 83 are inserted sothat the other ends of the second springs 83 at the driving source sidecan project to the driving source side.

The first springs 82 have a larger biasing force than the second springs83. The first springs 82 and the second springs 83 are coil springs, andthe first springs 82 are large-sized coil springs which have a largercoil diameter than the second springs 83. The second springs 83 have alonger free length than the first springs 82, and the other ends of thesecond springs 83 are held by the first retainer plate 84 so as to beprojectable from the second retainer plate 85 to the driving sourceside.

As illustrated in FIG. 7, the biasing unit 80 is attached to thetransmission case 11 by the first retainer plate 84 being supported atthe driving source side on both sides in the circumferential directionsof the bonding parts 71 of the oil channel forming member 70, and thesecond retainer plate 85 being supported at the anti-driving-source sideof the radially extended part 91 of the biasing force receiving member90.

The radially extended part 91 of the biasing force receiving member 90is formed to have the radial dimension which is smaller than the outerdiameter of the second retainer plate 85 and is substantially the sameas that of the second spring 83 so that it supports the second retainerplate 85, and supports the other end parts of the second springs 83inserted into the insertion holes 85 b of the second retainer plate 85.

The inner circumferential surface of the first cylinder part 22 of thehub member 20 is formed radially larger than the first retainer plate 84and the second retainer plate 85, and the biasing unit 80 is disposed atthe inner circumferential side of the first cylinder part 22. The innercircumferential surface of the first cylinder part 22 of the hub member20 projects radially inward at the driving source side to form a steppedpart 22 c, and a stop member 86 is attached to the anti-driving-sourceside of the stepped part 22 c.

The stop member 86 is formed radially smaller than the radially extendedpart 91 of the biasing force receiving member 90 and larger than thesecond retainer plate 85 so that it stops the second retainer plate 85when the second retainer plate 85 is moved to the driving source side byreceiving the biasing force of the biasing member 81.

The piston 50 is adjusted to be located at an immediately-before-contactposition where it is immediately before contacting the plurality offriction plates 40 when the second retainer plate 85 supported by thebiasing force receiving member 90 contacts the stop member 86. Theimmediately-before-contact position of the piston 50 is suitablyadjusted between the released position where the plurality of frictionplates 40 become in the released state and the zero clearance positionwhere the plurality of friction plates 40 become in the zero clearancestate. The stop member 86 may be comprised of a plurality of stopmembers, each formed in an arc shape having a given length in thecircumferential directions and is separated from one another in thecircumferential directions, or may be formed an annular shape coveringthe entire circumference.

When the second retainer plate 85 contacts the biasing force receivingmember 90, the biasing force receiving member 90 receives the biasingforce only from the first springs 82 in the engaging direction. Thepiston 50 is adjusted to be located at the zero clearance position whenthe first springs 82 become the free length.

Thus, the biasing unit 80 is constructed so that the first springs 82make the biasing force act in the engaging direction on the piston 50through the biasing force receiving member 90 from the released positionto the immediately-before-contact position, and the second springs 83make the biasing force act in the engaging direction on the piston 50through the biasing force receiving member 90 from the released positionto the zero clearance position. Thus, the brake BR2 can move the piston50 to the zero clearance position with sufficient response and withsufficient accuracy.

Then, when the hydraulic pressure for engagement is supplied to thehydraulic chamber 61 for engagement while the piston 50 is at the zeroclearance position, accordingly, the piston 50 pushes to move theplurality of friction plates 40 to the engaged position where theplurality of friction plates 40 become in the engaged state in which theplurality of friction plates 40 cannot be relatively rotatable by beingpinched between the piston 50 and a holding part 21 a which projects tothe anti-driving-source side from the vertical wall part 21 of the hubmember 20.

On the other hand, when the hydraulic pressure for engagement isreleased from the hydraulic chamber 61 for engagement and the hydraulicpressure for release is supplied to the hydraulic chamber 62 for releasewhile the piston 50 is at the engaged position, the piston 50 is biasedin the releasing direction and is moved to the zero clearance position.

Further, the piston 50 is biased in the releasing direction against thebiasing forces of the first springs 82, and is moved to theimmediately-before-contact position. Subsequently, the piston 50 isbiased in the releasing direction against the biasing forces of thefirst springs 82 and the second springs 83, and is moved the releasedposition.

In this automatic transmission 10, the hydraulic chamber 60 is disposedat a different radial position from the biasing member 81 but axiallyoverlaps with the biasing member 81. In detail, the hydraulic chamber 61for engagement and the hydraulic chamber 62 for release are disposed atdifferent radial positions from the first springs 82 and the secondsprings 83 but axially overlap with the first springs 82 and the secondsprings 83. Thus, the automatic transmission can be shortened in theaxial directions.

Next, the supply oil channel which supplies the hydraulic fluid to thebrake BR2 is described. A supply oil channel L1 for engagement whichsupplies the hydraulic fluid for engagement to the hydraulic chamber 61for engagement of the brake BR2, and a supply oil channel L2 for releasewhich supplies the hydraulic fluid for release to the hydraulic chamber62 for release of the brake BR2 are formed in the transmission case 11,the hub member 20, and the oil channel forming member 70. A supply oilchannel L3 for lubrication which supplies the hydraulic fluid forlubrication to the friction plates 40 is formed in the transmission case11 and the hub member 20.

As illustrated in FIG. 5, the supply oil channel L1 for engagement iscomprised of a radial oil channel 102 which is provided in the verticalwall part 21 of the hub member 20 and extends radially, an axial oilchannel 103 which is provided in the first cylinder part 22 of the hubmember 20, extends axially, and is coupled to the radial oil channel102, an axial oil channel 104 which is provided in the bonding parts 71of the oil channel forming member 70, extends axially, and is coupled tothe axial oil channel 103, a radial oil channel 105 which is provided inthe bonding parts 71, the coupling parts 73, and the hydraulic chamberforming part 72 for engagement of the oil channel forming member 70,extends radially, and is coupled to the axial oil channel 104, and anaxial oil channel 106 which is provided in the hydraulic chamber formingpart 72 for engagement of the oil channel forming member 70, extendsaxially, is coupled to the radial oil channel 105, and opens to thehydraulic chamber 61 for engagement.

The radial oil channel 102 of the hub member 20 is coupled to a radialoil channel 101 which is provided in the transmission case 11 andextends radially. The radial oil channel 101 leads to a control valveunit (not illustrated) disposed below the transmission case 11. Thecontrol valve unit supplies a given hydraulic pressure for engagement bysupplying the hydraulic fluid for engagement to the hydraulic chamber 61for engagement through the radial oil channel 101 and the supply oilchannel L1 for engagement.

The radial oil channel 105 of the oil channel forming member 70 isformed so as to extend radially inward from the outer circumferentialsurface of the bonding part 71 of the oil channel forming member 70, anda closing member 75 which closes the opening of the radial oil channel105 is attached to the outer circumferential surface of the bonding part71.

As illustrated in FIG. 6, the supply oil channel L2 for release iscomprised of a radial oil channel 112 which is provided in the verticalwall part 21 of the hub member 20 and extends radially, an axial oilchannel 113 which is provided in the first cylinder part 22 of the hubmember 20, extends axially, and is coupled to the radial oil channel112, and a radial oil channel 114 which is provided in the firstcylinder part 22, the vertical wall part 21, and the second cylinderpart 23 of the hub member 20, extends radially, is coupled to the axialoil channel 113, and opens to the hydraulic chamber 62 for release.

The radial oil channel 112 of the hub member 20 is coupled to a radialoil channel 111 which is provided in the transmission case 11 andextends radially. The radial oil channel 111 leads to the control valveunit. The control valve unit supplies a given hydraulic pressure forrelease by supplying the hydraulic fluid for release to the hydraulicchamber 62 for release through the radial oil channel 111 and the supplyoil channel L2 for release.

The radial oil channel 114 of the hub member 20 is formed so as toextend radially inward from the outer circumferential surface of thefirst cylinder part 22 of the hub member 20, and closing members 25which close the openings of radial oil channel 114 are attached to theouter circumferential surface of the first cylinder part 22.

The axial oil channel 113 of the hub member 20 is formed so as to extendaxially to the driving source side from an end face on theanti-driving-source side of the first cylinder part 22 of the hub member20, and the fastening bolts 74 are attached to the end face on theanti-driving-source side of the first cylinder part 22, as the closingmembers which close the openings of the axial oil channel 113.

The threaded holes 22 b are formed in the openings of the axial oilchannel 113 on the anti-driving-source side of the first cylinder part22. The fastening bolts 74 which couple the oil channel forming member70 to the hub member 20 are threadedly engaged with the threaded holes22 b, and the openings of the axial oil channel 113 are closed by usingthe fastening bolts 74. As the fastening bolt 74, a sealed bolt in whichthe outer circumferential surface of the thread part 74 b is coveredwith the sealant is used.

Thus, since the fastening members 74 coupling the oil channel formingmember 70 function as the closing members which close the openings ofthe axial oil channel 113 of the first cylinder part 22, the openings ofthe axial oil channel 113 can be closed without the number of componentsbeing increased.

As illustrated in FIG. 3, the supply oil channel L3 for lubrication iscomprised of a radial oil channel 122 which is formed in the verticalwall part 21 of the hub member 20 and extends radially, circumferentialoil channels 123 which are formed in the vertical wall part 21 of thehub member 20, arcuately extend in the circumferential directions, andare coupled to the radial oil channel 122, axial oil channels 124 whichare formed in the first cylinder part 22 of the hub member 20, extendaxially, and are coupled to the circumferential oil channels 123, radialoil channels 125 which are formed in the first cylinder part 22 of thehub member 20, extend radially, are coupled to the axial oil channels124, and open to the outer circumferential surface of the first cylinderpart 22 of the hub member 20.

A plurality of radial oil channels 125 which open to the outercircumferential surface of the first cylinder part 22 are formed with aninterval therebetween in the axial directions so that they open to therespective teeth parts of the spline 22 a. Preferably, the radial oilchannels 125 are formed so as to open to tip ends of the teeth parts ofthe spline 22 a. Note that the radial oil channels 125 may be formed soas to open to the bottoms of the teeth parts of the spline 22 a.

The radial oil channel 122 of the hub member 20 is coupled to a radialoil channel 121 which is provided in the transmission case 11 andextends radially. The radial oil channel 121 leads to the control valveunit. The control valve unit can supply the hydraulic fluid forlubrication to the plurality of friction plates 40 through the radialoil channel 121 and the supply oil channel L3 for lubrication.

In this brake BR2, the hydraulic fluid for lubrication is supplied tothe friction plates 40 from the outer circumferential surface of thefirst cylinder part 22 of the hub member 20 as the inner stationarymember to cool the generated heat of the friction plates 40. Thehydraulic fluid for lubrication supplied to the friction plates 40 movesto the inner circumferential surface of the cylinder part 31 of the drummember 30 as the outer rotary member, and moves in the axial directionsby the rotation of the cylinder part 31 of the drum member 30, andthereby stagnation of the hydraulic fluid for lubrication is prevented.

The axial oil channels 124 of the hub member 20 are formed so as toextend axially to the driving source side from an end face on theanti-driving-source side of the first cylinder part 22 of the hub member20, and closing members 26 which close the openings of the axial oilchannels 124 are attached to the end face on the anti-driving-sourceside of the first cylinder part 22.

In this automatic transmission 10, as illustrated in FIG. 10, the radialoil channels 102, 112, and 122 which respectively constitute the supplyoil channel L1 for engagement, the supply oil channel L2 for release,and the supply oil channel L3 for lubrication are sequentially disposedin the circumferential directions at a lower side of the transmissioncase 11. The circumferential oil channels 123 which constitute thesupply oil channel L3 for lubrication are coupled to the radial oilchannel 122, and arcuately extend in the circumferential directions awayfrom the radial oil channel 112, and extend to the opposite side of theradial oil channel 102 from the radial oil channel 112.

The supply oil channel L3 for lubrication is provided with a pluralityof (specifically, five) axial oil channels 124 coupled to thecircumferential oil channels 123, which are substantially equallyseparated from each other in the circumferential directions, and theaxial oil channels 124 are coupled to a plurality of radial oil channels125 which open to the outer circumferential surface of the firstcylinder part 22 of the hub member 20, respectively.

As illustrated in FIG. 3, the openings of two axial oil channels 124which constitute the supply oil channel L3 for lubrication are closed byattaching the closing members 26 to the end face on theanti-driving-source side of the first cylinder part 22. The openings onthe anti-driving-source side of three axial oil channels 124 whichconstitute the supply oil channel L3 for lubrication are closed by usingthe fastening bolts 74 which couple the oil channel forming member 70 tothe hub member 20.

As illustrated in FIG. 4, axial oil channels 124′ which constitute thesupply oil channel L3 for lubrication are formed so as to extend axiallyto the driving source side from an end face on the anti-driving-sourceside of the first cylinder part 22 of the hub member 20, and thefastening bolts 74 are attached as the closing members which closeopenings of the axial oil channels 124′ to the end face at theanti-driving-source side of the first cylinder part 22.

The threaded holes 22 b are formed in the openings of the axial oilchannels 124′ on the anti-driving-source side of the first cylinder part22 of the hub member 20. The fastening bolts 74 which couple the oilchannel forming member 70 to the hub member 20 are threadedly engagedwith the threaded holes 22 b, and the openings of the axial oil channels124′ are closed by using the fastening bolts 74. As the fastening bolt74, a sealed bolt in which the outer circumferential surface of thethread part 74 b is covered with the sealant is used.

Thus, since the fastening members 74 coupling the oil channel formingmember 70 function as the closing members which close the openings ofthe axial oil channels 124′ of the first cylinder part 22, the openingsof the axial oil channels 124′ can be closed without the number ofcomponents being increased.

As illustrated in FIG. 10, the axial oil channels 103, 113, and 124which constitute the supply oil channel L1 for engagement, the supplyoil channel L2 for release, and the supply oil channel L3 forlubrication, respectively are formed at different circumferentialpositions, and are formed in boss parts 22 d which project radiallyinward from the inner circumferential surface of the first cylinder part22 of the hub member 20. As illustrated in FIG. 11, cutoff parts 84 cand 85 c are formed in the first retainer plate 84 and the secondretainer plate 85 of the biasing unit 80, corresponding to the bossparts 22 d, respectively.

Next, operation of the brake BR2 constructed as above is described. FIG.13 is a cross-sectional view illustrating the brake in the releasedstate, FIG. 14 is a cross-sectional view illustrating the brake in theimmediately-before-contact state, FIG. 15 is a cross-sectional viewillustrating the brake in the zero clearance state, and FIG. 16 is across-sectional view illustrating the brake in the engaged state. FIGS.13-16 are enlarged views illustrating substantial parts of the brake inFIG. 7.

In FIG. 13, the released state of the brake BR2 where the piston 50 islocated at the released position in which the plurality of frictionplates 40 become in the released state is illustrated. This releasedstate of the brake BR2 is achieved by releasing the hydraulic pressurefor engagement from the hydraulic chamber 61 for engagement, andsupplying the hydraulic pressure for release to the hydraulic chamber 62for release, to compress the first springs 82 and the second springs 83through the biasing force receiving member 90 and move the piston 50 inthe releasing direction, that is, toward the anti-driving-source side.

When engaging the brake BR2, the hydraulic pressure for release isreleased from the hydraulic chamber 62 for release in the released stateillustrated in FIG. 13, and as illustrated in FIG. 14, the piston 50 ismoved in the engaging direction, that is, toward the driving source sideby receiving the biasing forces of the first springs 82 and the secondsprings 83 through the biasing force receiving member 90 until thesecond retainer plate 85 contacts the stop member 86, the piston 50 thenreaches the immediately-before-contact position where the piston 50 isat a location just before contacting the plurality of friction plates40, and thereby the brake BR2 becomes in the immediately-before-contactstate.

When the second retainer plate 85 contacts the stop member 86 in theimmediately-before-contact state, as illustrated in FIG. 15, the piston50 is moved in the engaging direction by receiving the biasing force ofthe second springs 83 through the biasing force receiving member 90until the second springs 83 reach the free length thereof, and thepiston 50 is then located at the zero clearance position where thepiston 50 contacts or substantially contacts the friction plates 40without pressing the plurality of friction plates 40 to make thefriction plates 40 become in the zero clearance state, and thereby thebrake BR2 becomes in the zero clearance state.

When the hydraulic pressure for engagement is supplied to the hydraulicchamber 61 for engagement in the zero clearance state, as illustrated inFIG. 16, the piston 50 is biased and moved in the engaging direction bythe hydraulic pressure for engagement supplied to the hydraulic chamber61 for engagement, the piston 50 then pushes the plurality of frictionplates 40 and reaches the engaged position where the plurality offriction plates 40 are unable to relatively rotate, and thereby thebrake BR2 becomes in the engaged state.

On the other hand, when releasing the brake BR2, in the engaged stateillustrated in FIG. 16, the hydraulic pressure for engagement isreleased from the hydraulic chamber 61 for engagement and the hydraulicpressure for release is supplied to the hydraulic chamber 62 forrelease, and the piston 50 is then biased and moved in the releasingdirection, that is toward the anti-driving-source side by the hydraulicpressure for release supplied to the hydraulic chamber 62 for release tobecome in the released state illustrated in FIG. 13 through the zeroclearance state illustrated in FIG. 15 and theimmediately-before-contact state illustrated in FIG. 14.

In this brake BR2, the piston 50 can be moved with sufficient responseby the first springs 82 and the second springs 83 from the releasedposition to the immediately-before-contact position, and can be movedwith sufficient accuracy by the first springs 82 from theimmediately-before-contact position to the zero clearance position.

As described above, the slip control of the brake BR2 is carried outwhen the vehicle starts traveling. When engaging the brake BR2, afterthe plurality of friction plates 40 are made into the slip state bysupplying the oil pressure lower than the hydraulic pressure forengagement to the hydraulic chamber 61 for engagement, the hydraulicpressure for engagement is supplied to the hydraulic chamber 61 forengagement to engage the plurality of friction plates 40. On the otherhand, when releasing the brake BR2, after the plurality of frictionplates 40 are made into the slip state by supplying the oil pressurelower than the hydraulic pressure for release to the hydraulic chamber62 for release, the hydraulic pressure for release is supplied to thehydraulic chamber 62 for release to disengage the plurality of frictionplates 40.

When engaging and releasing the brake BR2, the hydraulic fluid forlubrication is supplied to the plurality of friction plates 40 throughthe supply oil channel L3 for lubrication, and when the slip control ofthe brake BR2 is carried out, the hydraulic fluid for lubrication issupplied to the plurality of friction plates 40 through the supply oilchannel L3 for lubrication.

Thus, the automatic transmission 10 according to this embodimentincludes the friction engaging element BR2, which is provided with theplurality of friction plates 40, the piston 50 which causes theplurality of friction plates 40 to engage with each other, and thebiasing member 81 which biases the piston 50. The biasing member 81includes the first biasing member 82 which exerts the biasing force onthe piston 50 from the released position to theimmediately-before-contact position in the engaging direction, and thesecond biasing member 83 which exerts the biasing force on the piston 50from the released position to the zero clearance position in theengaging direction. The first biasing member 82 and the second biasingmember 83 are disposed at a different circumferential position from eachother but radially overlap with each other.

Thus, in the automatic transmission 10 including the friction engagingelement BR2 having the biasing member 81 which biases the piston 50,when the biasing member 81 includes the first biasing member 82 whichexerts the biasing force from the released position to theimmediately-before-contact position, and the second biasing member 83which exerts the biasing force from the released position to the zeroclearance position, compared to a case where first biasing member 82 andthe second biasing member 83 are not overlapped radially, the automatictransmission can be shortened in radial dimensions and thereby it can beradially downsized.

Since the biasing member 81 includes the first biasing member 82 whichexerts the biasing force from the released position to theimmediately-before-contact position, and the second biasing member 83which exerts the biasing force from the released position to the zeroclearance position, the piston 50 can be moved with sufficient responsefrom the released position to the immediately-before-contact position bythe first biasing member 82 in addition to the second biasing member 83,and the piston 50 can be moved with sufficient accuracy from theimmediately-before-contact position to the zero clearance position bythe second biasing member 83, and thereby the piston 50 can be movedwith sufficient response and with sufficient accuracy to the zeroclearance position. In particular, the first biasing member 82 isconfigured to exert a larger biasing force than the second biasingmember 83, and thereby the piston 50 can be moved with sufficientresponse and with sufficient accuracy to the zero clearance position.

Since the piston 50 can be moved with sufficient accuracy to the zeroclearance position, a shock caused by the piston 50 being located on theengagement side of the zero clearance position to press the frictionplates 40, and a slowing of the response caused by the piston 50 beinglocated on the releasing side of the zero clearance position, can beprevented.

Moreover, the hydraulic chamber 62 for release is formed to have asmaller outer diameter than the hydraulic chamber 61 for engagement, andthe biasing force receiving member 90 which is coupled to the piston 50and receives the biasing force of the biasing member 81 is disposed atthe outer circumferential side of the hydraulic chamber 62 for release.Thus, the biasing force receiving member 90 can be coupled to the piston50 at the outer circumferential side of the hydraulic chamber 62 forrelease of which the outer diameter is formed smaller than the hydraulicchamber 61 for engagement, thereby effectively utilizing the space atthe outer circumferential side of the hydraulic chamber 62 for release.

Moreover, the hydraulic chamber 60 to which the hydraulic fluid whichbiases the piston 50 is supplied is disposed at a position axiallyoverlaps with the first biasing member 82 and the second biasing member83 and a different radial position from the first biasing member 82 andthe second biasing member 83. Thus, compared to a case where thehydraulic chamber 60 is not overlapped axially with the first and secondbiasing members 82 and 83, the automatic transmission 10 can beshortened in axial dimensions and thereby it can be axially downsized.

The automatic transmission 10 includes the first retainer plate 84 whichholds one ends of the first biasing member 82 and the second biasingmember 83 and the second retainer plate 85 which holds the other end ofthe first biasing member 82 and has the insertion hole 85 b into whichthe second biasing member 83 is inserted. The piston 50 is biased by thesecond biasing member 83 projected on the second retainer plate 85 tothe direction opposite from the first retainer plate from theimmediately-before-contact position to the zero clearance position.Thus, the piston 50 can be moved with a comparatively simple structurefrom the released position to the immediately-before-contact position bythe first biasing member 82 and the second biasing member 83, and thepiston 50 can be moved from the immediately-before-contact position tothe zero clearance position by the second biasing member 83.

The present disclosure is not limited to the illustrated embodiment andvarious improvements and various design changes are possible withoutdeparting from the subject matter of the present disclosure.

As described above, according to the present disclosure, since theautomatic transmission provided with the friction engaging elementhaving the biasing member which biases the piston is radially downsizedwhen the biasing member includes the first biasing member and the secondbiasing member, it may also be suitably utilized in the manufacturingtechnology field of automatic transmissions and/or vehicles mounted withthe automatic transmissions.

It should be understood that the embodiments herein are illustrative andnot restrictive, since the scope of the invention is defined by theappended claims rather than by the description preceding them, and allchanges that fall within metes and bounds of the claims, or equivalenceof such metes and bounds thereof, are therefore intended to be embracedby the claims.

DESCRIPTION OF REFERENCE CHARACTERS

-   10 Automatic Transmission-   11 Transmission Case-   40 Friction Plate-   50 Piston-   60 Hydraulic Chamber-   61 Hydraulic Chamber for Engagement-   62 Hydraulic Chamber for Release-   81 Biasing Member (Spring)-   82 First Biasing Member (First Spring)-   83 Second Biasing Member (Second Spring)-   84 First Retainer Plate-   85 Second Retainer Plate-   85 b Insertion Hole-   90 Biasing Force Receiving Member-   BR2 Second Brake (Friction Engaging Element)

What is claimed is:
 1. An automatic transmission comprising a frictionengaging element, the friction engaging element including: a pluralityof friction plates disposed inside a transmission case; a pistonconfigured to cause the plurality of friction plates to be engaged witheach other; and a spring configured to bias the piston, the springincluding a first spring and a second spring with a longer free lengththan the first spring, and the first spring and the second spring beingaligned in circumferential directions of the transmission case.
 2. Theautomatic transmission of claim 1, further comprising a hydraulicchamber to which hydraulic fluid is supplied, the hydraulic fluidbiasing the piston, wherein the hydraulic chamber includes a hydraulicchamber for engagement to which hydraulic fluid to bias the piston in anengaging direction is supplied to cause the plurality of friction platesto be engaged with each other, and a hydraulic chamber for releasedisposed at an opposite side of the piston from the hydraulic chamberfor engagement, the hydraulic chamber for release being supplied withhydraulic fluid to bias the piston in a releasing direction oppositefrom the engaging direction, wherein the hydraulic chamber for releaseis formed to have a smaller outer diameter than the hydraulic chamberfor engagement, and wherein a biasing force receiving member configuredto receive the biasing force of the spring is coupled to the piston andis disposed at an outer circumferential side of the hydraulic chamberfor release.
 3. The automatic transmission of claim 2, wherein thehydraulic chamber is disposed so as to overlap with each of the firstspring and the second spring in a radially internal and externalrelationship.
 4. The automatic transmission of claim 1, furthercomprising a hydraulic chamber to which hydraulic fluid is supplied, thehydraulic fluid biasing the piston, wherein the hydraulic chamber isdisposed so as to overlap with each of the first spring and the secondspring in a radially internal and external relationship.
 5. Theautomatic transmission of claim 1, further comprising: a first retainerplate holding a first end part of the first spring and a first end partof the second spring; and a second retainer plate, disposed so as to beaxially separated from the first retainer plate, holding a second endpart of the first spring, and having an insertion hole into which thesecond spring is inserted so that a second end part of the second springis projectable in a direction opposite from the first retainer plate. 6.The automatic transmission of claim 2, further comprising: a firstretainer plate holding a first end part of the first spring and a firstend part of the second spring; and a second retainer plate, disposed soas to be axially separated from the first retainer plate, holding asecond end part of the first spring, and having an insertion hole intowhich the second spring is inserted so that a second end part of thesecond spring is projectable in a direction opposite from the firstretainer plate.
 7. The automatic transmission of claim 3, furthercomprising: a first retainer plate holding a first end part of the firstspring and a first end part of the second spring; and a second retainerplate, disposed so as to be axially separated from the first retainerplate, holding a second end part of the first spring, and having aninsertion hole into which the second spring is inserted so that a secondend part of the second spring is projectable in a direction oppositefrom the first retainer plate.
 8. The automatic transmission of claim 4,further comprising: a first retainer plate holding a first end part ofthe first spring and a first end part of the second spring; and a secondretainer plate, disposed so as to be axially separated from the firstretainer plate, holding a second end part of the first spring, andhaving an insertion hole into which the second spring is inserted sothat a second end part of the second spring is projectable in adirection opposite from the first retainer plate.
 9. An automatictransmission comprising a friction engaging element, the frictionengaging element including: a plurality of friction plates disposedinside a transmission case; a piston configured to cause the pluralityof friction plates to be engaged with each other; and a springconfigured to bias the piston in an engaging direction to cause theplurality of friction plates to be engaged with each other, the springincluding a first spring and a second spring with a longer free lengththan the first spring, and the first spring and the second spring beingaligned in circumferential directions of the transmission case.